FKAMP-6: Parse AMP CDN Urls to decide whether to redirect to HTTP or HTTPS

FKAMP-5 implemented detection of iframe elements referencing cdn.ampproject.org.

Currently the logic, if one's detected is

- Strip the URL fragment
- Split on /
- Take element 5 as the domain name
- Take element 6 onwards as the path
- prepend https://
- Redirect to that value

In an ideal world, all origin sites would be HTTPS, but realistically that's not necessarily true. So, we really should make sure we trigger a redirect to the correct protocol.

Looking at Google's AMP cache documentation, it looks like we can tell from the URL though. If position 4 == s then the origin is HTTPS, if not it's HTTP

Because /s follows the /c prefix, the document will be fetched from example.com using TLS.

So, we should include a check and set the protocol accordingly